1. Field of the Invention
The present invention relates to a screw having a shaft having an outer radial surface and a free end region, and at least one thread arranged on the shaft circumferentially surrounding the shaft at least in some regions, and having a radial outer profile, two side flanks, a thread pilot located in the free region of the shaft, and a plurality of recesses provided in the thread and opening radially outwardly, with the recesses each having a first limiting wall adjacent to the thread pilot and a second limiting wall remote from the thread pilot. A plurality of cutting bodies formed of a material having a hardness greater than a hardness of the thread is located in the recesses formed in the thread, with each cutting body having a longitudinal axis and an outer surface surrounding the axis at a distance from the axis.
2. Description of the Prior Art
A screw, which is described above, is screwed in a borehole that was preliminary formed in a constructional component, with the thread of the screw forming or tapping a counter-thread or an undercut in the constructional component.
For outside applications, the screws, such as concrete screws, advantageously, are formed of a corrosion-resistant steels. However, steel materials often have a hardness that is not sufficient for tapping a counter-thread in a mineral constructional component, and the thread of the screw wears off during the screw being screwed in. Toughening of the screw or, e.g., of the thread, which is formed integrally with the shaft, is possible only to a limited extent when a corrosion-resistant material is used for forming the screw.
German Publication DE 198 52 338 A1 discloses a screw which is formed of a stainless steel and the shaft and thread of which are provided with a plurality of receiving bores in form of recesses which extend radially with respect to the longitudinal axis of the screw and in which cutting bodies in form of cylindrical cutting pin inserts of a hardened steel are received. The cutting pin inserts facilitate cutting-in even in hard mineral constructional components such as, e.g. concrete or brickwork. The recesses have, in the thread, a first limiting wall adjacent to the thread pilot, and a second limiting wall remote from the thread pilot. The limiting walls have a profile for receiving, at least in some regions, the outer surface of cutting pin inserts which have a circular cross-section. The cutting pin inserts are retained in the receiving recesses in the shaft formlockingly in the screw in direction and frictionally in the radial direction.
One of the drawbacks of the screw described above consists in that all of the cutting pin inserts project radially beyond the cross-sectional projection surface of the thread to the same extent. In order to set such thread-tapping screws, a high screw-in torque is required, in particular, for screws with a small diameter.
Another drawback of the above-described screw consists in that its manufacture is very expensive. This is because a number of cutting pin inserts need be separately knock in a number of the receiving recesses and then finished.
Accordingly, an object of the present invention is to provide a thread-tapping screw that permits to improve the setting of the screw in a hard constructional component, e.g., concrete.
Another object of the present invention is to provide a thread-tapping screw that can be simply manufactured.